This is really about defence in depth

Imagine that the data you want to protect - your most valuable digital asset - is at the heart of an onion. Around that data, you need layers of defence - infrastructure, devices - they all form layers. And each of these layers, which have security of their own, add to the overall defence of your valuable data. After all, it’s not just the data that needs securing, it’s the container of that data, too. You can never be truly secure by focusing on just one element.

By focusing on the whole ‘onion’ you create a multi-layered and multi-dimensional overall security solution.